As we strengthen the process sandbox, the security of our IPC mechanisms because more and more paramount. Like the rendering engine itself, existing C++ IPC struggles with all the memory unsafety that using C++ entails. I imagine this would look like adding Rust codegen to the IPDL codegen, as well as adding a small C++ shim to plug the Rust APIs into the existing IProtocol class hierarchy. We'd want to make sure this worked well for both top-level protocols and for sub-protocols.